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Monday, 17 February 2014

TWO PHASE LOCKING PROTOCOL(PESSIMISTIC APPROACH)A transaction follows the two phase locking protocol, if all the locking operations precede the first unlock operation in the transaction. There are two phases in the Schedule. These are:1.) GROWING PHASE- During which all the locks are requested.2.) SHRINKING PHASE- During which all the locks are released.Initially a transaction is in the growing phase. The transaction acquires locks as needed. Once the transaction releases a lock, it enters the shrinking phase and it can issue no more lock requests.For Example: Lock X(A) Read(A) A:= A-200 GROWING PHASE Write(A) Lock X(B) --------- Unlock(A) Read(B) B:=B+200 SHRINKING PHASE Write(B) Unlock(B)PROBLEMS WITH TWO PHASE LOCKING PROTOCOL:-1.) Deadlock2.) Cascading Roll-Back